1. Field of the Invention
The present invention relates to a pulley, and more particularly to a resin formed pulley having improved strength.
2. Background Information
Recently, in order to decrease the weight of crank pulleys or the like, and to reduce the production costs thereof, attempts have been made to form the pulleys from a synthetic resin material.
One example of such a crank pulley is illustrated in FIGS. 1 and 2. A main body 10 consists of a circular plate made from a synthetic resin material. A flange portion 12 is formed in the outer periphery of the main body 10 for retaining a belt(not shown). A boss 14 composed of a metallic cylindrical body is embedded in an axially central portion of the main body 10. The boss 14 is adapted to be secured to a crank shaft (not shown). A plurality of metallic cylindrical inserts 16 of a smaller diameter are embedded in the main body 10 so as to penetrate its plate portion at regular intervals in the circumferential direction of the main body 10, thereby defining service holes which are used in drawing the pulley out from the crank shaft.
A plurality of projections 20 are formed in the outer peripheral surface of the boss 14 so as to project to the inside of the plate portion of the main body 10 at regular intervals in the circumferential direction of the boss 14. These projections 20 serve to improve the joining strength of the boss 14 and the main body 10.
However, when large torsional vibrations are exerted on the pulley from the crank shaft, the synthetic resin material of the main body 10 is liable to break in the portions A (FIG. 2) which contact the corners of the top end of each projection 20, and thus, sufficient strength joining the boss 14 and the main body 10 cannot be obtained.
When a torsional vibration torque of 1000 N.m is input to a pulley where the distance (R1) between the axial center of the boss 14 and the top end of each projection 20 is 22 mm, the interval(D) between two adjacent projections 20 is 4.2 mm, and the thickness (T1) of each projection 20 (FIG. 1) is 5.5 mm, the stress generated in the portion A is as large as 1968.times.10.sup.6 N/m.sup.2.
Furthermore, when the pulley with the above-described construction is drawn out from the crank shaft by using the service holes 18, the synthetic resin material composing the main body 10 is liable to break in the vicinity of each cylindrical metallic insert 16.
In addition, if the atmospheric temperature rises considerably, the synthetic resin material composing the main body 10 separates from the projections 20, as shown by the chain line in FIG. 3, due to the difference between the coefficient of linear thermal expansion of the synthetic resin material and of the metal composing the projections 20. Hence, looseness occurs in the vicinity of the boss 14.